Hydrogen embrittlement inhibitors for organic compositions

ABSTRACT

HYDROGEN EMBRITTLEMENT OF STEEL IN CONTACT WITH AQUEOUS SOLUTIONS OF ORGANIC COMPOUNDS WHICH HYDROLYZE TO AMMONIA OR AMINES IS INHIBITED BY INCORPORATION IN THE AQUEOUS ORGANIC SOLUTION A MINOR AMOUNT OF A DIMERIC ACID HAVING FROM ABOUT 32 TO 44 CARBON ATOMS, E.G., DILINOLEIC ACID, AND AN ORGANO-PHOSPHORUS COMPOSITION WHICH IS THE REACTION PRODUCT OF A MONOALKYL PHENOL OR A DIALKYL PHENOL AND PHOSPHOROUS PENTOXIDE.

United States Patent O 3,740,336 HYDROGEN EMBRITTLEMENT INHIBITORS FOR ORGANIC COMPOSITIONS Frank H. Langenfeld, St. Louis, and Wilson W. Overall, Watson Woods, Mo., assignors to Monsanto Company, St. Louis, M0. N Drawing. Filed Dec. 23, 1970, Ser. No. 101,192

Int. Cl. (309k 3/18 U.S. Cl. 252-70 11 Claims ABSTRACT OF THE DISCLOSURE Hydrogen embrittlement of steel in contact with aqueous solutions of organic compounds which hydrolyze to ammonia or amines is inhibited by incorporation in the aqueous organic solution a minor amount of a dimeric acid having from about 32 to 44 carbon atoms, e.g., dilinoleic acid, and an organo-phosphorus composition which is the reaction product of a monoalkyl phenol or a dialkyl phenol and phosphorus pentoxide.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to hydrogen embrittlement inhibitors for organic compositions, to a method for preventing hydrogen embrittlement of metals in contact with aqueous solutions of certain organic compounds which hydrolyze to ammonia or amines, and particularly to the inhibition of amide-based deicer or ice melting compositions to prevent embrittlement of steel contacted therewith.

Statement of prior art Hydrogen embrittlement of steel occurs when free hydrogen atoms adsorbed on the metal surface diffuse into the metal by intercrystalline or interstitial diffusion. Once in the steel the hydrogen may remain in atomic form or, upon reaching an interstitial void of larger than atomic dimensions, may combine to form internal pockets of hydrogen gas. Hydrogen is found to permeate preferentially in stressed regions and to enter the voids nearest the stressed regions.

The diffusion of hydrogen into the steel is accompained by the formation of internal gas pockets, initiation and promotion of cracks in high stress areas, and certain other phenomena which induce the condition characterized by delayed brittle failure of the steel and by reduced ability of the steel to support sustained loads.

Hydrogen embrittlement is induced in steel in a number of ways including for example, acid pickling, cathodic cleaning, electroplating, electrochemical machining, heating in moist atmospheres or hydrogen-bearing atmospheres, exposure to moisture under corrosive conditions and exposure to hydrogen at elevated temperature and pressures.

Embrittlement of steels is known to occur in bodycentered cubic microstructures such as exist in tempered martensite, 'bainite, lamellar pearlite and spheroidized structures, but fully austenitic steels are found to be quite resistant to such embrittlement. In general, higher strength steels, i.e., above about 200,000 p.s.i. ultimate tensile strength, are more susceptible to this type of failure although embrittlement has been found in steels having strength levels of 60,000 psi. or lower. The composition of the steel is not an important factor in hydrogen embrittlement, and no alloying element either substitutional or interstitial has been truly effective in retarding hydrogen induced delayed brittle failure.

Because hydrogen embrittlement limits the use of martensitic steels at high strength levels it is a particularly severe problem in the aircraft industry where AISI 4340, a chromium-nickel-molybdenum steel, is extensively used in aircraft parts such as landing gear assemblies. In the aircraft industry, most hydrogen embrittlement problems are related to cleaning and electroplating operations prior to assembly, and to exposure to embrittlement promoting atmospheres and conditions during the use of the aircraft. One potential source of hydrogen embrittlement of aircraft steel, and the source with which this invention is particularly concerned, is the embrittlement induced when the steel is contacted with aqueous amide compositions used as runway and aircraft deicers. A pre ferred deicer composition comprises a mixture of formamide and urea. We have discovered that aqueous solutions of urea-formamide promote hydrogen embrittlement of high strength steels found in aircraft landing gear assemblies and elsewhere. We have also found that certain conventional hydrogen embrittlement inhibitors used for aqueous inorganic systems, such as sodium nitrate, although effective in preventing hydrogen embrittlement, render the deicer corrosive and unsuitable for use in this application. Other known inhibitors such as diethylaniline for example are simply not effective in the urea-formamide-water system.

SUMMARY It is an object of this invention to provide a method for preventing hydrogen embrittlement of metals in contact with aqueous solutions of organic compounds which hydrolyze to form ammonia or amines. It is a further and particular object of this invention to provide an inhibited urea-formamide deicing composition which does not promote hydrogen embrittlement or corrosion of high strength steels.

The objects of this invention are accomplished and embrittlement of steel in contact with aqueous solutions of amides is essentially prevented by incorporating in the aqueous solution an inhibitor comprising a mixture of a dimeric acid having from about 32 to 44 carbon atoms, e.g. dilinoleic acid, and an organo-phosphorus composition which is the reaction product of an alkyl substituted phenol such as diamylphenol and P 0 From about 0.05 to 0.50 percent by weight of the inhibitor based upon the weight of organic components of the aqueous solution is a usually effective concentration.

The invention is particularly useful in preventing hydrogen embrittlement of high strength steel and cadmium plated steel in aircraft landing gear assemblies which are contacted with deicer compositions comprising urea and formamide when these compositions are applied to runways or the aircraft itself.

DESCRIPTION OF PREFERRED EMBODIMENTS The organic solutions with which this invention is concerned comprise aqueous solutions of organic compounds which are soluble in water and which hydrolyze to form ammonia or amines. Examples of such organics include amides such as formamide, acetamide, propionamide, butyramide, valeramide, and the like; alkyl amides such as methylformamide, ethylformamide, dimethylformamide, diethylformamide, methylacetamide, ethylacetamide, dimethylacetamide, diethylacetamide, methylpropionamide, ethylpropionamide, dimethylpropionamide, diethylpropionamide, methylbutyramide, ethylbutyramide, dimethylbutyramide, diethylbutyramide, methylvaleramide, ethylvaleramide, dimethylvaleramide, diethylvaleramide, and the like; and derivatives of carbonic acids such as C C., alkyl carbamates, urea, C -C alkyl substituted area, carbamidine, and the like. The amides and derivatives of carbonic acid useful in the present invention are those having less than about 10 carbon atoms.

Of particular interest are aqueous solutions of urea, formamide, and mixtures thereof, which have been suggested for use as deicers for airport runways and for aircraft. In a preferred deicer composition, from about 50 to 75 parts formamide and 20 to 45 parts urea are dissolved with the inhibitor in 5 to 15 parts water to form a solution which is then sprayed on the runways or the aircraft. If desired, the inhibitor may be added to a ureaformamide mixture in the absence of water to form a deicer concentrate which can be mixed with the desired amount of water when used. Additional dilution of the formamide-urea composition occurs as the ice is melted, and the solution which finally contacts the steel aircraft parts and promotes embrittlement may have a concentration of from 100% to 10% or less of the original concentration of the applied deicer solution. In evaluating the effectiveness of various inhibitors in formamide-urea solutions, a standard solution containing about 75% water and 25% organic material was selected as being representative of concentrations contacting aircraft under most typical operating conditions.

The dimeric acids useful in the present invention are the condensation product of two conjugated or nonconjugated polyolefinic fatty acids having from about 16 to 22 carbon atoms. The acids are monocyclic or dicyclic depending upon the degree of unsaturation of the monocarboxylic acid from which the dimeric acids are prepared. Thus, octadecadienoic acids polymerize to monocyclic hydroaromatic dicarboxylic acids while octadecatrienoic acids polymerize to bicyclic hydroaromatic dicarboxylic acids. Preferred dimeric acids are the dimers of dienoic acid trienoic acids containing about 18 carbon atoms, and dilinoleic acid is particularly preferred as a readily available, commercially attractive acid.

While the dimeric acids can be used in pure form, the commercial acids usually contain only about 85 percent dimeric acid, the balance being trimeric and higher polymeric acids. Such compositions are found to be suitable for use in the inhibitor compositions described herein, and reference hereinafter to dimeric acid includes these commercial acids as well as pure acids.

The organo-phosphorus compositions useful in the present invention are the reaction products of monoalkyl phenols or dialkyl phenols and P The alkyl substituents the preferably C to C hydrocarbons. The organo-phosphorus compositions are prepared by reacting in a conventional manner from about 2 to 6 moles of substituted phenol with one mole of P 0 to yield a product comprising a mixture of phosphorus acids, esters, and acid esters which is best defined simply as an organo-phosphorus reaction product of the substituted phenol and P 0 Examples of typical commercially available substituted phenols which can be used in this reaction include butyl and dibutyl phenol, amyl and diamyl phenol, and octyl and dioctyl phenol. Mixtures of these and other C, to C alkyl substituted phenols can also be used. A preferred organo-phosphorus composition is the reaction product of about 3 moles of di-t-amyl phenol and 1 mole of P 0 The inhibitors of the present invention comprise mixtures of dimeric acid and organo-phosphorus compositions which preferably contain at least 5 percent of either and up to 95 percent of the other, and more preferably contain a major proportion of at least about 50 percent of dimeric acid as hereinbefore described and a minor proportion of at least about 5 percent of the organo-phosphorus composition as hereinbefore described. The inhibitor compositions may be employed in the aqueous organic solutions at concentrations as low as 0.01 percent by weight of the organic component of solution, with preferred concentrations being from about 0.05 to 0.50 percent by weight. Higher concentrations can of course be employed but in view of the low solubility of these inhibitors such higher concentrations are generally not effective to improve performance.

Several tests have been proposed for measuring hydrogen embrittlement of steels and for determining the effec tiveness of organic inhibitors in simultaneously reducing corrosion and embrittlement. It is possible for example, to calculate the ratio between hydrogen evolved and the hydrogen equivalent to the amount of metal dissolved during a corrosion test, and to determine by this ratio the extent to which hydrogen was adsorbed by the metal.

Another test, the Hydrogen Embrittlement Script Test, subjects steel in a stressed condition to an embrittling environment and records the time required for the steel to fail. This test is an effective tool for scanning a large number of inhibitor compositions at minimum expense. The corrosion rate for the solutions is easily determined by weight loss suffered by the strips during the embrittlement test. The test procedure and results are described in detail below.

EMBRITTLEMENT STRIP TEST This test is conducted by immersing standard test strips held in a stressed condition in a test solution until the strip fails. Test strips used in the following examples were standard HEP strips, a product of Associated Spring Corporation. HEP strips are flat springs of 1075 steel, /2 x 7 /2 inches by 25 mils thick, which have been cadmium plated and heat treated under controlled conditions to assure minimum variations in test results. Two small U-shaped tabs stamped into the strips approximately 3% inches from either end provide four high stress points at the legs of the tabs when the strip is bent into an arch. The dull cadmium plated test strips simulate cadmium plated steel commonly used in aircraft.

In conducting the test, five test strips are individually bent into an arch having a 3.5 inch base and placed into a device designed to restrain the ends of the strip in a fixed position. Care is taken in bending the strip to avoid inducing any kinking or localized bending or metal deformation which would result in decreasing the stress at the stress points formed by U-tabs.

The five test specimens are immersed in a bath of the solution to be evaluated for embrittlement characteristics. The bath and test specimens are allowed to stand quietly at room temperature. As the test specimens become embrittled they weaken and eventually fail. The time at which the third test strip breaks is taken as the mean time to failure and signals the end of the test. The mean time to failure can vary from a few hours to several days depending upon the nature of the test solution. Solutions in which the mean time to break was less than 5 days were considered to fail this test and to be unsatisfactory from an embrittlement standpoint. Solutions in which mean time failure occurred in 5 to 10 days were considered marginal while solutions which exceeded 10 days were considered satisfactory.

The following is a tabulation of data showing the mean time to break for several urea-formamide solutions containing different inhibitor compositions. All test data were obtained on the standard test strips and at room temperature. Compositions of the test solutions were given in weight percent.

TABLE I.-HYDROGEN EMBRITTLEMENT STRIP TEST Bare steel corrosion Time to Wt. percent organic compo- Wt. percent rate, mils/ failure, nents of solution inhibitor* yr. days 1- 19 formamide, 5 urea. None 7. 1 1 2 .do 0.04 A 2.3 8 60 0.40 A 60 0.04 NaNO 8.1 5 0.10 NaNOa-- 8.7 10 0.20 NaNOa--- 8.1 60 0.40 NaNOg-.. 4. 4 60 0.20 kerosene. 6.9 1

*Inhibitor, percent by weight of organic component of best solution.

NoTn.Inhibitor A=% commercial dilinoleie acid, 5% reaction product of '3 m. dl-t-amy l phenol plus '1 m. P205, kerosene.

Since Inhibitor A contained kerosene as a carrier for dilinoleio acid and organo-phosphorus composition,

Exp. 9 was conducted to determine what effect, if any, was attributable to the presence of the kerosene. It is apparent from these data, that no benefit is derived from the use of kerosene alone.

The embrittlement strip test data illustrate that while NaNO and the organic inhibitors of the present invention are essentially equivalent in preventing stress failure due to hydrogen embrittlement, the NaNO promotes increased corrosion of steel and is not suitable for use in the aqueous organic systems of this invention for that reason.

Corrosion rates for inhibited aqueous formamide-urea solutions were also determined on bare AISI 4130 steel and on dull cadmium plated AISI 4130 steel b immersing steel test coupons in various solutions at room temperature, and determining corrosion rate from weight loss after 72 hours. Corrosion data obtained for seven test solutions are presented in Table II below:

TABLE II.-CO RROSION TEST Corrosion rate, mg./in.'-

Cad. Wt. percent organic components Wt. percent Bare plated solution inhibitor" steel steel 3. 1 12. 2 4. 4. 9 2. 3 1. 9 0.9 1.0 0.7 d 6. 0 15. 3 16 -d0 0.40 NaNO 8. 6 15.

*Inhibitor, percent by weight of organic component of test solution Mg./in. Brt. cad. plated steel 0.53 Magnesium 0.52 Aluminum (Alclad 2024T3) 0.07 Aluminum (Alclad 7075T6) 0.09

These data further demonstrate the suitability of inhibited urea-formamide solutions for use in connection with aircraft deicing operations where metals such as magnesium and aluminum are common materials of construction. The two aluminum specimens identified above are common aircraft skin materials, and it is therefore imperative that any runway or aircraft chemical deicers be passive with respect to these materials.

The inhibitor compositions of the present invention are unique in their multifunctional ability to simultaneously inhibit hydrogen embrittlement and corrosion of steel and cadmium plated steel without adversely affecting magnesium or aluminum. The discovery of these inhibitors has made possible the practical application of a recognized superior ice melting composition which heretofore could not be used without causing serious damage to aircraft. The inhibited urea-formamide deicer composition fulfills a long felt need by the aircraft industry and airport control authorities for a means whereby ice and snow could be quickly, economically, and safely removed from airport runways and from the aircraft itself.

The use of these deicer compositions is not of course limited to applications in the aircraft industry, but can also be used on streets and highways, sidewalks, buildings, ships, and the like. Applications for inhibited aqueous amide solutions other than deicers are also made possible by the discovery of the inhibitors as disclosed herein.

Although the preceding examples have been directed to inhibition of aqueous formamide-urea solutions with dilinoleic acid and diamyl phenyl phosphate, these examples are provided by way of illustration of the invention,

and the invention is not intended to be limited thereby. Other dimeric acids, other phosphate esters, and other amide-based aqueous organic solutions, all hereinbefore described may be substituted for the specific compositions given in the examples, and are accordingly included within the scope of the present invention as defined by the claims following.

The embodiments in which an exclusive property or privilege is claimed are defined as follows 1. A composition consisting essentially of:

(A) one or more organic compounds selected from the group consisting of amides, C alkyl carbamates, urea, and C alkyl-substituted urea and carbamidine which are hydrolyzable to ammonia or amines; and

(B) a minor amount of at least about 0.01 percent by weight of (A) and suflicient to inhibit hydrogen embrittlement of steel contacted with aqueous solutions of (A) of a composition comprising from about 5 to 95 parts by weight of a dimeric acid having from about 32 to 44 carbon atoms and from about 95 to 5 parts by weight of a reaction product of 2 to -6 moles of a monoalkyl phenol or dialkyl phenol wherein the alkyl has from about 4 to 10 carbon atoms and one mole of phosphorus pentoxide.

2. A composition of claim 1 wherein the amount of (B) is from about 0.01 to 0.50 percent by weight of the organic compounds of (A).

3. A composition of claim 2 wherein the organic compounds are selected from the group consisting of urea,

' formamide, and mixtures thereof.

4. A composition of claim 3 wherein the dimeric acid is dilinoleic acid and the organo-phosphorus reaction product is of about 3 moles of diamyl phenol and 1 mole of phosphorus pentoxide.

S. A composition of claim 4 wherein (B) comprises a major proportion of at least 50 percent by weight of said dimeric acid and a minor proportion of at least 5 percent by weight of said organo-phosphorus reaction product.

6. A composition of claim 1 in aqueous solution containing from about 5 to percent by Weight water.

7. A composition of claim 5 in aqueous solution containing from about 5 to 95 percent by weight water.

8. A composition consisting essentially of:

(A) an aqueous solution of one or more organic compounds selected from. the group consisting of amides C alkyl carbamates, urea, and C alkyl-substituted urea and carbamidine which are hydroxylzable to ammonia or amines; and

(B) from 0.01 to 0.50 percent by weight of said organic compounds of a composition consisting essentially of from about 95 to 5 parts by weight of dimeric acid having from about 32 to 44 carbon atoms and from about '5 to 95 parts by weight of the reaction product of from 2 to 6 moles of a monoalkyl phenol or a dialkyl phenol wherein the alkyl has from about 4 to 10 carbon atoms and one mole of phosphorus pentoxide.

9. A composition of claim 8 wherein said organic compounds are selected from the group consisting of urea, formamide, and mixtures thereof.

10. A composition of claim 9 wherein (B) consists essentially of a major proportion of at least 50 percent by weight of dilinoleic acid and a minor proportion of at least 5 percent by weight of a reaction product of about 3 moles of diamyl phenol and 1 mole of phosphorus pentoxide.

11. A composition of claim 8 wherein (A) is an aqueous solution consisting essentially of from about 50 to 75 parts by weight formamide, 2-0 to 45 parts by weight urea, and 5 to 15 parts by weight water, and (B) consists essentially of a major proportion of at least 50 percent by weight of dilinoleic acid and a minor proportion of at least 5 percent by weight of a reaction product 8 of about 3 moles diamyl phenol and 1 mole phosphorus 3,624,243 11/1971 Scott et a1. 252-70 pentoxide. 3,630,913 12/1971 Scott et a1. 252-70 References Cited UNITED STATES PATENTS HERBERT B. GUYNN, Primary Examiner 3,282,731 11/1966 Hudson et a1. 134 41 5 PITLICK Assistant Examiner 3,310,497 3/1967 Hudson et a1. 252146 3,340,197 9/1967 Davidowich et a1. 252146 3,345,298 10/1967 Leeds et a1. 252-146 389 

